The present invention relates to hydraulic shock absorbers of the direct double acting tubular type, and in particular, to a shock absorber of this type having an improved piston compression valve assembly. The shock absorber of the present invention is especially useful on vehicles such as snowmobiles.
Direct double acting tubular shock absorbers utilized on vehicles usually comprise a cylinder attached to the axle of the vehicle and a piston in the cylinder which is connected with the frame so that when the vehicle passes over an uneven surface the piston and cylinder move relative to one another. The cylinder contains a suitable damping fluid which is transferred across the piston, and simultaneously a smaller amount, equivalent to the piston rod volumetric displacement, is expelled from one end of the cylinder into a reservoir when the axle and frame move toward one another, relatively. This action is referred to as the compression stroke of the shock absorber. In many instances the valving within the shock absorber is designed to restrict the flow of fluid across the piston and the fluid flow from piston rod volumetric displacement from the cylinder during the compression stroke and thus restrain the motion of the vehicle.
One design for controlling the flow of fluid across the piston during the compression stroke involves mounting an elastomeric o-ring piston seal on the piston body so that the exterior periphery of the o-ring engages the interior periphery of the cylinder to prevent fluid flow along the interior periphery of the cylinder. The upper surface of the o-ring engages a radially extending flange of a seal carrier member of L-shaped cross-sectional configuration. The seal carrier member has a cylindrical leg portion, the bottom surface of which is provided with one or more slots therein. The upper surface of the flange portion of the seal carrier abuts against a compression bypass spring mounted concentrically about the upper portion of the piston. The upper end of the bypass spring abuts against a piston stop. A seal protector ring is mounted on the piston body so that the upper face of the protector ring abuts the lower surface of the o-ring and with the o-ring covers the slots in the seal carrier.
During a compressive movement of the piston, hydraulic fluid in the compression chamber below the piston body is pressurized. At low compression speeds and forces the fluid pressure deflects (deforms) the o-ring and fluid passes radially inwardly through the slots in the bottom of the cylindrical leg portion of the carrier into the rebound chamber above of the piston body. As piston speed increases, the fluid pressure overcomes the load on the bypass spring, the o-ring and the seal carrier move upwardly on the piston, away from the protector ring, thus allowing fluid to pass radially inwardly around the entire annular axially extending space between the bottom of the cylindrical leg portion of the seal carrier and the protector ring into the rebound chamber above the piston body.
One problem with this prior art design is that a relatively large oil flow area is exposed with only a slight upward movement (as little as 0.02 inches) of the seal carrier. Once the seal carrier and o-ring separate from the protector ring exposing the large oil flow area, the bypass spring element of fluid control is essentially eliminated. Thus, the bypass spring element of fluid flow control essentially becomes an on/off switch.
Once the bypass spring element of the control is overcome by fluid pressure, fluid flow control is provided by restriction holes of fixed diameter in the piston body. The difficulty with such restriction holes is that fluid flow control is viscosity dependent rather than pressure dependent.
Since viscosity of the fluid decreases as shock temperature increases, hot thinner fluid will have lower resistance to passing through the piston fixed diameter restriction holes than cooler, thicker fluid. The lower resistance at elevated fluid temperatures will result in lowered pressure on the compression chamber side of the piston and the shock will therefore generate less compression ride control force at a given piston speed. This loss of ride control force at elevated fluid temperatures is known as "fade". Such fade may be significant in shock absorbers subject to temperature extremes, such as in snowmobiles, where the initial fluid temperature is at room temperature or below and may increase to 200.degree. F. or more during use.